1. Field of Invention
This invention regards a device for a hydraulic cutting tool for cutting of tubular objects beneath a water floor, e.g. beneath a seafloor.
The device is preferably used for cutting of casings in connection with the permanent plugging and abandonment of a well drilled under water, e.g. a petroleum well. After cutting, the cut-off pieces of casing may be removed from the water floor. Such a well may be completed at the water floor or above water, e.g. on a platform or another type of surface facility. In the latter case the well is connected to the surface facility via a riser. On the other hand, both types of wells are drilled under water and down into a water floor, and such a well is hereinafter termed an offshore well.
Said device may also be used in connection with the cutting of other types of tubular objects disposed in a water floor. Such an object may comprise a tubular pile or a caisson. As an example, tubular piles are used to anchor platforms and other offshore structures to a water floor. In this case, the piles are driven into the water floor, then to be fixed to appropriate fixing devices such as fixing brackets on the offshore structure in question.
2. Description of the Related Art
The invention is based on the cutting of casings, in particular compound casings, beneath a water floor upon abandonment of offshore wells. When cutting beneath a water floor, access from the outside of the pipes is impossible, making it necessary to perform the cutting from inside the casing. In this connection, known cutting devices and cutting methods are encumbered with a number of disadvantages and problems.
Cutting of said tubular objects under a water floor, including casings, piles and caissons, is normally carried out mechanically, hydraulically or through blasting.
As the invention comprises a hydraulic cutting tool that is known per se, and which is typically used for cutting of casings in an offshore well, the following discussion will only concern hydraulic cutting of casings according to prior art. This discussion also concerns those disadvantages of known hydraulic cutting techniques which the present invention seeks to remedy. This is also necessary in order to understand significant characteristics of the invention, as well as the problems which the invention seeks to remedy.
A well is normally composed of several casing strings arranged inside each other with decreasing diameters, where each smaller casing string extends deeper into the ground than the previous and larger casing string. In addition, one or more annuli between the casing strings may be completely or partially filled with set cement. Such casing strings are hereinafter only termed casings.
The cutting of casings beneath a water floor is carried out by means of a hydraulic cutting tool which is lowered into the well from a surface facility such as a platform, the cutting tool being lowered to the relevant cutting position in the innermost casing of the well. The cutting tool is equipped with a high pressure nozzle through which a concentrated jet of fluid exits at high speed, cutting through the casing and any annular cement. The exiting high speed jet normally has a diameter of 1–2 mm and is delivered at a very high pressure, for example 1000 bar. The cutting jet consists of a fluid, preferably water, mixed with an abrasive. The cutting fluid is hereinafter termed an abrasive fluid. According to prior art, such hydraulic cutting is carried out at water depths of up to 100 meters, and the cutting is often carried out 5 meters beneath the water floor. Moreover, the cutting method is relatively quick, requires little equipment, and may be carried out with a minimum risk of injury/damage to personnel, servicing means and any remaining downhole equipment, including well plugs that seal against any reservoir fluids.
In principle, a conventional hydraulic cutting system consists of a high pressure pump; a mixing device in which said fluid and abrasives are mixed; a cutting tool comprising among other things said high pressure nozzle, a high pressure line through which said abrasive fluid is pumped down to the cutting tool; at least one auxiliary line via which e.g. hydraulic and/or electrical driving power and/or hydraulic/electrical control and/or monitoring signals are transmitted to the cutting tool; and a hoisting device such as a wire winch for bringing the cutting tool into or out of the well. In addition, the cutting tool comprises an actuator, preferably hydraulically actuated, for fixing and possibly sealing the cutting tool in the casing in question; a rotating motor, preferably hydraulically actuated, for rotating the high pressure nozzle during the cutting; and various other known equipment such as sprockets, shafts, bearings, gears, clamping implements, gaskets, hydraulic cylinders and pistons, pipes, couplings, control units and monitoring equipment. Operation of said rotating motor and actuator depends among other things on there being auxiliary lines available through which said driving power and control and/or monitoring signals may be transmitted to the cutting tool.
From the surface facility and in the innermost casing of the well, the cutting tool, said high pressure line for abrasive fluid and said auxiliary lines are lowered to the cutting position beneath the water floor. Then the cutting tool is fixed against the wall of the casing in the working position by at least one associated hydraulically actuated and releasable anchoring device, e.g. a clamping jaw or a clamping claw. The cutting tool may also be equipped with at least one hydraulically actuated and releasable anchoring-and sealing device, e.g. at least one rubber elastic packing, which is pressed against the casing wall and separates two sections of the casing in a pressure tight manner. In the latter case, the anchoring device and the sealing device may be actuated by a common hydraulic actuator device driven and controlled by means of said auxiliary lines.
Hydraulic cutting is initiated by the abrasive fluid being pumped from said high pressure pump and down through said high pressure line to the cutting tool. The abrasive fluid is conducted further through the cutting tool to an angular and rotatable high pressure pipe, the free end of which is connected to said high pressure nozzle, the high pressure pipe and the nozzle projecting down from the cutting tool. By means of a rotating motor and suitable transmission means, said pipe and nozzle are rotated peripherally through at least one complete rotation (at least 360° angle) about the longitudinal axis of the casing. The high pressure pipe and the nozzle are rotated at an appropriate peripheral speed, and preferably in the horizontal plane, the cutting jet simultaneously cutting through one or more casings and any annular cement. In this connection at least one annulus may be completely or partially filled with set cement, liquid and/or air.
According to prior art, the hydraulic cutting is generally carried out in an environment consisting of the liquid normally present in the innermost casing, e.g. seawater. The cutting jet will therefore pass through a liquid between the nozzle outlet and the casing wall. However this results in a lot of the initial pressure energy of the cutting jet being lost through impact loss when the cutting jet collides with the liquid in the casing at high speed. In some cases the liquid filled casing is therefore arranged with a small pipe volume that is filled with air or nitrogen, the pipe volume being arranged immediately below the cutting tool and comprising the cutting site in question. Said air or nitrogen is hereinafter simply termed a gas. In principle, the cutting jet will thereby pass through gas instead of liquid, whereby said impact loss is reduced considerably. By so doing, a significantly greater share of the initial pressure energy of the cutting jet should be available for cutting the casings and any annular cement. In principle, it should then be possible to cut through pipes and any annular cement much more quickly, whereby any disruptive or damaging influential forces have considerably less time to affect the cutting result in a negative manner. Said influential forces may arise as a result of flow movements or hydrostatic pressure changes in the liquid column above the cutting tool. The influential forces may cause the cutting tool and the cutting jet exiting from it to be subjected to undesirable axial movement, which causes an undesirable reduction in cutting power and the precision of the cut. This may cause the continuity of the cutting to be interrupted and/or cause the resulting faces of the cut to form a discontinuous, e.g. helical, cut in stead of a continuous and circular cut. In both cases the cutting must be repeated. Such movement may also cause fluid leaks in the gaskets of the cutting tool, whereby seeping liquid flows into the cutting area in question, possibly reducing the impact force of the cutting jet.
In order to allow said pipe volume to be filled with said gas, the cutting tool must be connected to a compressor on the surface facility via a pressure line for gas. According to prior art, the cutting tool is also equipped with a short drain pipe running through the cutting tool. The upper end of the drain pipe is terminated immediately above the cutting tool, and the lower end of the pipe is terminated below the cutting depth in question. Moreover, the drain pipe is designed to be peripherally rotatable together with the high pressure pipe and the high pressure nozzle, to prevent the cutting jet from cutting off the drain pipe during rotation.
After the cutting tool according to prior art has been fixed in a pressure tight manner in the innermost casing of the well, pressurised gas is pumped into said pipe volume underneath the sealing means of the cutting tool via said pressure line. The gas is supplied at a pressure which is sufficient to force water in this pipe volume out through the short drain pipe in the cutting tool, to be mixed with the surrounding water immediately above the cutting tool. By so doing, the pipe volume comprising the cutting depth in question is filled with pressurised gas. During the cutting, pressurised gas is continuously pumped into this pipe volume.
Even though the known technique of hydraulic cutting in a gas filled environment is more efficient than cutting in liquid, the known technique of cutting in gas is also encumbered with considerable disadvantages. Among them is the fact that a continuous feed of pressurised gas via said small pipe volume will also entail a continuous outflow of pressurised gas at the top of said short drain pipe. Thus, gas bubbles will continuously rise and expand in the overlying liquid column of the casing. Expansion of gas bubbles in the liquid column may cause percussions or movements in the liquid column, and such influential forces may propagate downwards in the liquid column, possibly causing unwanted movement of the cutting tool during the cutting, cf. previous mention of this. Continuous outflow of gas immediately above the cutting tool also means that the gas pressure in the cutting area in question can not exceed the hydrostatic pressure at the outlet of the short drain pipe to any appreciable extent. Cutting in said gas filled volume is therefore carried out at a marginal gas overpressure. In addition, this gas overpressure will remain roughly unchanged even if the gas inflow rate to the pipe volume is increased. Instead, such an increase will cause a greater outflow of undesirable gas bubbles rising and expanding in the liquid column of the casing. In addition to these disadvantages, the marginal gas overpressure is also a considerable disadvantage to the hydraulic cutting. When the fluid jet cuts through casings and possibly annular cement, the marginal gas overpressure will be insufficient to prevent hydrostatically pressured liquid from the outside of the casing/casings from trickling into the gas filled casing volume via one or more cuts in said casing. Thus the cutting jet will collide with inflowing liquid, causing an impact loss to the cutting jet, which reduces the impact force of the cutting jet. This reduction in the inherent energy of the cutting jet is particularly disadvantageous when cutting through several consecutive casing sizes, as this loss of energy reduces the ability of the cutting jet to cut efficiently through all the casings and any associated annular cement.